Problem: 8 glue sticks cost $12.08. Which equation would help determine the cost of 9 glue sticks?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 8 glue sticks. We want to know the cost of 9 glue sticks. We can write the numbers of glue sticks as a proportion: $\dfrac{8}{9}$ We know 8 glue sticks costs $12.08. We can let $x$ represent the unknown cost of 9 glue sticks. The proportion of these costs can be expressed as: $\dfrac{\$12.08}{x}$ The cost changes along with the number of glue sticks purchased, and so the two proportions are equivalent. $\dfrac{8}{9} = \dfrac{\$12.08}{x}$